1. Field of the Invention
The present invention relates to a heating apparatus for an electromagnetic induction heating system. In particular, the present invention relates to a fusing apparatus using the heating apparatus and to an image forming apparatuses such as a copying machine, a facsimile or a printer that uses an electrostatic recording system.
2. Description of Related Art
Normally, fixing devices that use an electromagnetic induction heating system as a heating source changed to, for example, a halogen lamp, have come to be widely used in order to save energy and increase speed of fixing devices used for image forming apparatuses such as printers and copying machines. In related fixing devices that use an electromagnetic induction heating system, a magnetic field generated by a magnetic field generator acts on a heat generator with this eddy current causing this heat generator to generate Joule heat. As an example, this heating apparatus can be used as a fixing device of an image forming apparatus that heats images not yet fixed which are formed on a recording medium (such as transfer or an OHP sheet) by applying an image forming technique.
In order to shorten the heating time, fixing devices, which used the heating apparatus applying the electromagnetic induction heating system, utilized a thin conductive film layer with a small heat capacity. This film would generate an eddy current in the magnetic field of the magnetic excitation coil and thereby generate heat. However, when comparing the center and the end of the film in the lengthwise direction of the fixing belt, heat is lost to the bearing that supports the film at the end and in addition because the heat radiating surface area of the opening is large, the amount of heat radiating along the end becomes larger. Because of this, the temperature at the end of the fixing belt reduced compared to the temperature at the center of the fixing belt thereby making it impossible to supply enough heat energy to the recording material and the toner on the recording material at the end leading to a problem of offset that resulted in incomplete toner fixing at the end. Conventionally, in order to improve the phenomenon of temperature reductions at the end of these fixing belts, a composition was used in which the distance between the magnetic excitation coil and the heat generator was varied (for example Related Art 1). [Related Art 1] Japanese Patent Publication H9-26719
The length of the magnetic path in the composition that varies the distance between the magnetic excitation coil and the heat generator disclosed in Related Art 1 above becomes longer, increasing the magnetic resistance and thereby increasing the electric power supplied to the heat generator. Because the upper limit of the electric power that can be supplied from a power outlet to household appliances and office equipment (to an image forming apparatus in this invention) is restricted by laws and regulations, there are also restrictions on the amount of electric power that can be supplied to a magnetic excitation coil used for electromagnetic induction heating. Consequently, there are problems of the portion of the amount of electric power that effectively increased the magnetic resistance becoming a loss thereby making it impossible to supply enough electric power to a heat generator resulting in the time the temperature of the heat generator rises becoming slower.